Apologies Big Pete, I missed your post before. Water raises/lowers with startup/shutdown.
I hope this will satisfy your curiosity Merlyn and allow you to move back from the edge of your seat
As far as I can find out we do not have a smokemeter on board, so unfortunately I have not had the pleasure. Also we do not have a keel cooling system. We have a HT/LT system with plate coolers.
But I have a curious update!, I rejoined the vessel in the last week and the problems are very much reduced. The dirty water appears very similar to when we paid off.
According to our back to back crew no work was done on auxiliary engines.
Due to the colour of the water it was impossible to judge how much treatment chemical to use in the system before and as we were flushing often we believed it to be a waste. Also around this time we noticed the earth straps on both units were still disconnected and neatly tucked under the plates. We presume some contractor from drydock.
The disconnected earth strap to us would explain the dirty water from corrosion but not huge built up of air when the engine was started and warming up. (Yet once at operating temp system was all okay).
Just before our departure we calculated how much water was in the system and added the appropriate amount of chemical and reconnected the earth straps to both units. We did not notice a change in symptoms.
Rejoining the vessel, water was not as muddy from when we normally rejoin, and after a thorough flushing it is now possible to achieve okay analysis using our testing kit for nitrate ppm The water level does not raise on startup/warmup. We only flushed yesterday but by now the water is usually quite dirty. The photo attached shows current sample and I will try to get find a pic of the old dirty water.
We are still monitoring and are not convinced the above work solved this problem. There is an idea that with all the muddy/dirty water this somehow self-sealed an exhaust gas leak somewhere?
Anyone with an idea of how the above work carried out stopped the introduction of air, only at start/warmup of unit. Please post.
I was wondering if electrolysis was causing the water to break its bonds into oxygen and hydrogen where the oxygen rich environment accelerated the corrosion. But would have thought if this was possible it would continually happen and not at a temperature range of 65 to 70degC. Where running temp is 76 to 78deg.
Many thanks to all for your help, and to D windsor for his idea of checking earth straps.